As is well known in the art, it is an ordinary practice to make richer the air-fuel mixture to be combusted in an internal combustion engine than a normal or, for example, stoichiometric mixture under high-load conditions (such as full throttle or accelerating conditions) when the engine is required to produce increased power outputs.
Enrichment of the air-fuel mixture to be produced in an internal combustion engine is also required under high-speed operating conditions of the engine when the exhaust gases discharged from the engine tend to be heated excessively if the air/fuel ratio of the mixture to be combusted in the engine is maintained at the stoichiometric value.
In a conventional fuel-injection internal combustion engine, the fuel injection valve of the engine is therefore controlled to achieve an increased fuel injection amount under high-load or high-speed operating conditions of the engine. The adjustment of the fuel injection amount in this internal combustion engine is effected in such a manner that the fuel injection amount is increased by a predetermined value when the output speed of the engine, the intake air flow rate in the air induction pipe of the engine, or the opening degree of the throttle valve provided in the induction pipe of the engine is increased beyond a predetermined value.
In the fuel injection amount control system of this nature, however, a problem is encountered in that the fuel injection amount cannot be adjusted satisfactorily for all the operating conditions of the engine. This is partly because of the fact that the fuel injection amount is adjusted by a fixed value and partly because of the limited degree of freedom allowed of determining the boundary between the conditions in which the adjustment is to be effected and the conditions in which the adjustment is not to be effected.
In fuel-injection internal combustion engines, especially those for automotive use, the operating conditions of the engines are liable to variation over a broad range and to sudden changes and, for this reason, proper control of the fuel injection amount is more necessary than in internal combustion engines for other applications. For enhancement of the exhaust emission control efficiency, the driveability and the fuel economy in a fuel-injection internal combustion engine, furthermore, it is required that the fuel injection amount should be controlled more precisely than those controlled by existing fuel injection rate control systems.
The present invention contemplates resolution of these problems encountered in prior-art fuel injection control systems for fuel-injection internal combustion engines and, accordingly, it is an object of the present invention to provide a fuel injection control system capable of controlling the fuel injection amount in a fuel-injection combustion engine properly and precisely for various operating conditions of the engine by the aid of an electronic control circuit such as a microcomputer.